This invention regards a two-part telescopic tensioner for connection to a riser extending between a borehole and a floating installation on a subsea oil or gas field, where the purpose of the tensioner is to maintain tension in the riser, partly through taking up the rapid vertical movements of the floating installation, and partly through compensating for the slow changes in difference in level between the top of the borehole with its seabed installation, and the floating installation.
Waves and wind cause rapid changes in the level difference between the seabed and the floating installation used for offshore exploration or production of hydrocarbons.
Slow changes are caused by tidal changes, changes in the load on the installation, trimming of the installation for adjustment of freeboard according to forecast changes in the weather and in the event of horizontal drift.
The main function of a telescopic riser unit is to ensure that the upper part of the riser is able to telescope without any leakage of the liquids and/or gases that are being conveyed through the pipe. The telescopic unit may co-operate with a separate system for riser tensioning, or the telescopic unit may comprise integrated hydraulic cylinders that through co-operation with pumps and accumulators ensure that the required tension is maintained in the riser.
In waters having great tidal variations and/or a large design wave height, the telescopic units used today have a length of stroke of up to 70 feet (21,34 metres).
The use of single telescopic units dimensioned to accommodate both rapid and slow predictable variations has several disadvantages. It leads to                a) movement of an unnecessarily large dynamic mass;        b) wear and tear and a requirement for maintenance on large units; and        c) a requirement for several sizes.        